let _Vue = null;

export default class VueRouter {
  static install (Vue) {
    // 1. 判断当前插件是否已经被安装
    if (VueRouter.install.installed) {
      return;
    }

    VueRouter.install.installed = true;
    // 2. 把Vue构造函数记录到全局变量
    _Vue = Vue;
    // 3. 把创建Vue 实例时候传入的router对象注入到Vue实力上
    _Vue.mixin({
      beforeCreate () {
        if (this.$options.router) {
          _Vue.prototype.$router = this.$options.router;
          this.$options.router.init();
        }
      }
    })
  }

  constructor (options) {
    this.options = options;
    this.routeMap = {};
    this.data = _Vue.observable({
      current: '/'
    });
  }

  init () {
    this.createRouteMap();
    this.initComponents(_Vue);
    this.initEvent();
  }

  createRouteMap () {
    // 遍历所有的路由规则，把路由规则解析成键值对的形式 存储到routeMap中
    this.options.routes.forEach(route => {
      this.routeMap[route.path] = route.component;
    })
  }

  initComponents (Vue) {
    Vue.component('router-link', {
      props: {
        to: String
      },
      render (h) {
        return h('a', { 
          attrs: { href: this.to },
          on: { click: this.clickHandler }
        }, [ this.$slots.default ])
      },
      methods: {
        clickHandler (e) {
          e.preventDefault();
          history.pushState({}, '', this.to);
          this.$router.data.current = this.to;
        }
      }
    })

    const self = this;
    Vue.component('router-view', {
      render (h) {
        const component = self.routeMap[self.data.current];

        return h(component);
      }
    })
  }

  initEvent () {
    window.addEventListener('popstate', () => {
      this.data.current = window.location.pathname;
    })
  }
}